First of all, knit yourself a tension square to make sure you are knitting to the suggested sizes given for the pattern. Then you need to measure yourself.
When it comes to scarves, blankets, and even hats, sizing is pretty straightforward. But when you’re ready for your first sweater, things get a little more complicated. You might see S, M, L, etc., and then there are the actual measurements.
The first type is not very useful. It will tell you what range of sizes an item comes in (e.g. a sweater that comes in XS, S, M, L, XL and XXL has a range of six sizes, while a sweater that comes in S, M, L only has a range of three sizes), which can give you some clues about fit and patterning but these sizes really shouldn’t be used to determine which size you’re going to make.
What’s really important are the actual finished measurements, generally given as a chest or bust measurement. So if you want your sweater to be a big boxy cardigan, you probably want to choose one with a finished measurement 4-6″ larger than your body measurements (this is called 'positive ease'). Looking for a more figure-hugging sweater? Choose a measurement that’s 2-4″ smaller than your body measurements ('negative ease').